diff --git a/aurp/aurp.go b/aurp/aurp.go
index d49d83e..0382595 100644
--- a/aurp/aurp.go
+++ b/aurp/aurp.go
@@ -259,8 +259,10 @@ func (ot *OptionTuple) WriteTo(w io.Writer) (int64, error) {
 	}
 
 	a := acc(w)
-	a.write8(uint8(len(ot.Data) + 1))
-	a.write8(uint8(ot.Type))
+	a.write([]byte{
+		byte(len(ot.Data) + 1),
+		byte(ot.Type),
+	})
 	a.write(ot.Data)
 	return a.ret()
 }
@@ -371,6 +373,36 @@ func parseOpenReq(p []byte) (*OpenReqPacket, error) {
 	}, nil
 }
 
+// OpenRsp is used to respond to Open-Req.
+type OpenRspPacket struct {
+	*Header
+
+	RateOrErrCode int16
+	Options       Options
+}
+
+func (p *OpenRspPacket) WriteTo(w io.Writer) (int64, error) {
+	a := acc(w)
+	a.writeTo(p.Header)
+	a.write16(uint16(p.RateOrErrCode))
+	a.writeTo(p.Options)
+	return a.ret()
+}
+
+func parseOpenRsp(p []byte) (*OpenRspPacket, error) {
+	if len(p) < 3 {
+		return nil, fmt.Errorf("insufficient input length %d for Open-Rsp packet", len(p))
+	}
+	opts, err := parseOptions(p[2:])
+	if err != nil {
+		return nil, err
+	}
+	return &OpenRspPacket{
+		RateOrErrCode: int16(binary.BigEndian.Uint16(p[:2])),
+		Options:       opts,
+	}, nil
+}
+
 // ParsePacket parses the body of a UDP packet for a domain header, and then
 // based on the packet type, an AURP-Tr header, an AURP routing header, and
 // then a particular packet type.
@@ -419,6 +451,9 @@ func ParsePacket(p []byte) (Packet, error) {
 			}
 			orsp.Header = h
 			return orsp, nil
+
+		default:
+			return nil, fmt.Errorf("unknown routing packet command code %d", h.CommandCode)
 		}
 
 	default:
diff --git a/aurp/wtacc.go b/aurp/wtacc.go
index a42c4de..0bbabd7 100644
--- a/aurp/wtacc.go
+++ b/aurp/wtacc.go
@@ -1,13 +1,12 @@
 package aurp
 
 import (
-	"encoding/binary"
 	"io"
 )
 
 // wtacc is a helper for io.WriterTo implementations.
-// It sacrifices early returns for a shorter syntax. However, it refuses to
-// continue writing to the destination writer after detecting an error.
+// It sacrifices early returns for a shorter syntax. However, it won't continue
+// writing to the destination writer after detecting an error.
 type wtacc struct {
 	w   io.Writer
 	n   int64
@@ -21,25 +20,17 @@ func (a *wtacc) ret() (int64, error) {
 }
 
 func (a *wtacc) write8(x uint8) {
-	if a.err != nil {
-		return
-	}
-	_, a.err = a.w.Write([]byte{x})
-	if a.err != nil {
-		return
-	}
-	a.n++
+	a.write([]byte{x})
 }
 
 func (a *wtacc) write16(x uint16) {
-	if a.err != nil {
-		return
-	}
-	a.err = binary.Write(a.w, binary.BigEndian, x)
-	if a.err != nil {
-		return
-	}
-	a.n += 2
+	// Could do this with:
+	//     binary.Write(a.w, binary.BigEndian, x)
+	// - but don't wanna
+	a.write([]byte{
+		byte(x >> 8),
+		byte(x & 0xff),
+	})
 }
 
 func (a *wtacc) write(b []byte) {